Method for producing a positive electrode composite material for Na ion battery

ABSTRACT

The present invention relates to a method for producing a positive electrode composite material comprising at least one Na-based positive electrode active material and Na 3 P for a battery using sodium ions as electrochemical vector, to a positive electrode comprising such a positive electrode composite material, and to the Na-ion battery comprising such a positive electrode.

The present invention relates to a method for producing a positiveelectrode composite material comprising at least one Na-based positiveelectrode active material and Na₃P for a battery using sodium ions aselectrochemical vector, to a positive electrode comprising such apositive electrode composite material, and to the Na-ion batterycomprising such a positive electrode.

There are batteries called lithium-ion (Na-ion) batteries that use acarbon derivative at the negative electrode. The carbon derivative maybe a “soft” carbon, containing primarily sp² carbon atoms, a “hard”carbon containing primarily sp³ carbon atoms, or an intermediate varietyof carbon in which coexist variable proportions of sp² carbon atoms andsp³ carbon atoms. The carbon derivative may also be natural graphite orartificial graphite, optionally covered with ungraphitized carbon whichprotects against exfoliation during electrochemical operation. The majordrawback of these materials is the consumption of a part of the current,and hence of lithium ions originating from the positive electrode,during the first charge, the result of this being the formation, on thenegative electrode of a protective layer, called passivating layer (orSEI layer), which prevents subsequent reaction of the electrolyte on thenegative electrode into which the lithium is inserted. This phenomenongives rise to a decrease in the energy density of the battery, since thelithium rendered unusable is withdrawn from the positive-electrodematerial, which has a low specific capacity (90-210 mAh·g⁻¹). Inpractice, between 5% and 25% of the initial capacity is lost in thisway.

Also known is the use as negative-electrode material of transition metalfluorides, oxides, sulfides, nitrides, or phosphides, or of lithium andtransition metal fluorides, oxides, sulfides, nitrides, or phosphides,said transition metals being selected from T^(M)=V, Cr, Mn, Fe, Co, Ni,Cu, and Zn. By reaction with the lithium, these materials form atwo-phase system comprising the metal T^(M) and, respectively, LiF,Li₂O, Li₂S, Li₃N, or Li₃P, in the form of a mixture of particles havingnanometric sizes. These reactions are called “conversion” reactions andexhibit a substantial capacity (400 to 800 mAh·g⁻¹). The low size of thegrains in the two-phase mixture formed endows this reaction with acertain reversibility, since transport by diffusion/migration need beensured only over distances of a few nanometers. However, the electrodesof this type, whose design and implementation are simple, have thedrawback of an irreversible first-cycle capacity of 30% to 45%, therebyinhibiting their commercial development.

In addition, large-scale application of Li ion batteries, are facingchallenges related to scarcity of lithium resources and high cost.

The most appealing alternative to Li-ion batteries regarding chemicalelement abundance and cost is by all means sodium. Batteries usingsodium ions as electrochemical in place of lithium ions are employed foruse in place of lithium in applications where the stored energy densityis less critical than for portable electronics or automotive transport,more particularly for the management of renewable energies. Suchawareness has prompted the revival of the Na-ion battery concept withintense activity devoted to the search of highly performing electrodematerial. As in Li-ion batteries, regarding Na-ion negative electrode,carbon is the most attractive together with the use of Na-alloys withamong them the Na_(x)Sb phases being the most performing one. Turningnow to positive electrodes, polyanionic compounds such as NaFePO₄,Na₃V₂(PO₄)₂F₃, Na₂Fe₂(SO₄)₃, Na₃V₂(PO₄)₃ or layered compounds such asNa-based nickel manganese cobalt oxide phases (NMC phases) such asNaNi_(1/3)Mn_(1/3)Co_(1/3)O₂ or P2-layered phases such asNa_(2/3)[Fe_(1/2)Mn_(1/2)]O₂ phase which contain about 0.7 Na ions (Nat)per formula unit, are presently most studied candidates. The “hardcarbons”, which can also be used as negative-electrode material forNa-ions batteries, can give reversible Na⁺ insertions of the order of200 mAh·g⁻¹, but here as well the formation of a passivating layer isnecessary and represents a loss of 15% to 40% on the first cycle.

Research has then been carried out into means of compensating this lossof sodium, which in practice diminishes the energy density, since it istechnically not possible to remove the fraction of positive-electrodematerial which has served to form the passivating layer, said fractionremaining as a dead weight during the subsequent operation of thebattery.

The best theoretical way to fight cycle irreversible capacity in Na-ionbatteries would be somewhat similar to what has been done for Sielectrodes in Li-ion batteries in which Si in contact with a thin Lifoil by pressure leads in situ in the cell to the formation of Li_(x)Sionce the electrolyte is added, Li_(x)Si then compensate to loss of Liions during the formation of the passivating layer of the negativeelectrode. However, this solution cannot be applied to Na-ion batteriesdue to the practical limitation of making Na foils.

It is also not possible to simply add metallic sodium in positiveelectrode composite materials because metallic sodium is very reactivewith moisture. Na is very difficult to deal with because sticking tospatula, tweezers and so on. Moreover, only bulk Na is available withoutany powder form existed.

From EP-0 966 769 the addition is known of an alkali metal oxo carbon tothe active material of a positive electrode in a battery which operatesby circulation of lithium ions between the electrodes, for the purposeof at least partly remedying the loss in capacity during the 1stcycling, resulting from the formation of a passivating layer. However,during the 1st cycling of the battery, oxidation of the oxo carbonproduces anion radicals which are soluble in an electrolyte, the effectof this being to degrade the negative electrode. There is indeedimprovement in the initial capacity, but at the expense of the lifetimeof the battery.

Proposals have also been made to add NaN₃ as sacrificial salt inpositive electrode composite materials comprisingNa_(2/3)[Fe_(1/2)Mn_(1/2)]O₂ as positive electrode active material,acetylene black (AB) as electronic conducting agent and polyvinylidenedifluoride (PVDF) as binding agent in a ratioNa_(2/3)[Fe_(1/2)Mn_(1/2)]O₂:NaN₃:AB:PVDF=75:5:15:10, said compositebeing coated on an aluminum foil and Na metal being used as negativeelectrode with a glass microfiber used as separator. 1 M NaClO₄ in asolvent mixture (ethylenecarbonate/prolylenecarbonate 1:1) was used aselectrolyte in the electrochemical cell testing (Singh G. et al.,Electrochemistry Communications, 2013, 37, 61-63). This positiveelectrode composite material was tested in comparison with a positiveelectrode composite material which was identical except that it did notcomprise NaN₃. For the testing cell in which the positive electrodematerial did not comprise NaN₃, the first charge capacity was observedto be 139 mAh/g, which corresponds to the extraction of about 0.45sodium ions from the structure. While discharging, more sodium wasinserted back in the structure, hence a high discharge capacity of 197mAh/g was obtained. First cycle missing capacity is thus 59 mAh/g.However in the same configuration, if Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂ wasused as positive electrode active material with hard carbon or any othermaterial without Na as negative electrode active material, then theapparent capacity of Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂ would have not beenachieved because carbon is not a Na reservoir and the need to built aSEI, making this unfeasible. By comparison, when NaN₃ was present at anamount of 5 w % in the positive electrode composite material, themissing capacity was reduced to 27 mAh/g, demonstrating an improvementof about 50%. This enhancement is due to the decomposition of NaN₃ intoNa and N₂ during the first cycle. However, the use of NaN₃ assacrificial salt in the positive electrode composite material toalleviate irreversible capacities in Na-ion batteries is not totallysatisfactory because the presence of N₃ ⁻ into the electrode materialare prejudicial to the performances of the battery. In addition, inNaN₃, the use of 3 N atoms are needed to bring only one Na atom to thecomposite, which has the drawback of adding weight to the correspondingelectrode composite material and thus to the Na-ion batteryincorporating such an electrode. Finally, the production of N₂ volatilespecies during the first charge of the battery is prejudicial to thecohesion of the electrode material.

There is therefore still a need of providing Na-ion batteries exhibitingimproved performances in terms of irreversible capacity during the firstcharge, while being at the same time safe and not too heavy.

Thus the aim of the present invention is to provide a battery which usessodium ions as electrochemical vector, with its operation enhanced byreduction in the loss of capacity during the first discharge/chargecycle.

This aim is achieved by a method for producing a positive electrodecomposite material for a battery using sodium ions as electrochemicalvector, wherein said method comprises at least one step of mixing apowder of Na₃P with a powder of at least one positive-electrode activematerial capable of inserting sodium ions reversibly, said step ofmixing being carried out in a dry atmosphere and without heating.

Thanks to this method, it is now possible to prepare a positiveelectrode composite material comprising an intimate mixture of Na₃P andat least one Na-based positive-electrode active material capable ofinserting sodium ions reversibly. When said composite is then used asactive material of a positive electrode, Na₃P contained in the compositematerial liberates Na ions to compensate for the irreversibility of thenegative carbon electrode, hence increasing the overall energy density(a reduction of more than 50% of the irreversible capacity is obtained).Moreover, the P atoms remaining after the first charge of the batteryare in solid form into the electrode composite material rather than involatile form (as compared to the use of NaN₃). Another advantage isthat P has a molecular weight of 31 g and is able to bring 3 Na ionswhile, in the case of NaN₃, 3 N atoms are needed to bring only 1 Na ion.

According to the present invention, the expression “Na-basedpositive-electrode active material capable of inserting sodium ionsreversibly” refers to P2 type layered crystalline Na-phases comprisingNa and at least one oxide of at least one element selected from thegroup consisting of Fe, Mn, Co, Ni, P, S, Mn, V, Ti, and/or others solidcrystalline Na-phases.

The Na-based positive-electrode active material capable of insertingsodium ions reversibly may be selected from:

-   -   lamellar fluorophosphates Na₂TPO₄F in which T represents a        divalent element selected from Fe, Mn, Co, and Ni, and in which        T may be replaced partially by Mg or Zn;    -   fluorosulfates NaT′SO₄F in which T′ represents at least one        element selected from Fe, Mn, Co, and Ni, a part of T′ being        optionally replaced by Mg, and a part of the sulfate groups SO₄        ²⁻ of which is optionally replaced by the isosteric and        iso-charge group PO₃F²⁻;    -   polysulfides Na₂S_(n) (1≤n≤6), and sodium salts of        dimercaptothiadiazole and of dimercaptooxazole;    -   dithiocarbamates Na[CS₂NR′R″] in which each of the groups R′ and        R″ represents a methyl, ethyl, or propyl radical, or else R′ and        R″ form a ring (for example, pyrrolidine or morpholine);    -   Na₂Fe₂(SO₄)₃;    -   NaFePO₄, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃;    -   P2 type layered crystalline Na-phases selected from        Na_(0.67)Fe_(0.5)Mn_(0.5)O₂, Na_(0.67)MnO₂, Na_(0.74)CoO₂,        Na_(0.67)Co_(0.67)Mn_(0.33)O₂, Na_(0.67)Ni_(0.25)Mn_(0.75)O₂ and        Na_(0.67)Ni_(1/3)Mn_(2/3)O₂;    -   NaNi_(1/3)Mn_(1/3)Co_(1/3)O₂.

Among these Na-based positive electrode active materials, one can moreparticularly mention Na₂Fe₂(SO₄)₃, NaFePO₄, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃,Na_(0.67)Fe_(0.5)Mn_(0.5)O₂, Na_(0.67)MnO₂, Na_(0.74)CoO₂,Na_(0.67)Co_(0.67)Mn_(0.31)O₂, Na_(0.67)Ni_(1/3)Mn_(2/3)O₂ andNaNi_(1/3)Mn_(1/3)Co_(1/3)O₂.

According to a preferred embodiment of the present invention, theNa-based positive electrode active material is Na₂Fe₂(SO₄)₃ orNa_(0.67)Fe_(0.5)Mn_(0.5)O₂ or Na₃V₂(PO₄)₂F₃ or Na₃V₂(PO₄)₃.

The amount of Na₃P preferably varies from 2 w % to 40 w %, with regardto the weight of Na-based positive electrode active material. Inparticular, the amount of Na₃P can be adjusted depending on how many Naare required to compensate the irreversible capacities.

Within the meaning of the present invention, the expression “dryatmosphere” means that the atmosphere is anhydrous or moisture-free.Preferably, the atmosphere contains less than 20 ppm of water.

Within the meaning of the present invention, the expression “withoutheating” means that the method is implemented without any externalsource of heating.

In other terms, it is possible that the mixing step involves a heating(or temporary heating) of the reactants during said mixing, for exampledue to friction or exothermic reactions. However, the heating isinherent to said mixing step and not to an external source of heating.

According to a particulate embodiment of the present invention, themixing step can be performed in the presence of an electronicallyconducting agent in powder form, such as carbon powder.

In that case, the powder of electronically conductive agent can be addedat any time of the mixing step. The amount of electronically conductiveagent can vary from about 2 to 40 weight % with regard to the totalamount of powder materials (powders of Na₃P and positive electrodeactive material), and more preferably from about 5 to 15 weight %.

The mixing step can be carried out by any means allowing the obtainingof an intimate mixture of Na₃P with the Na-based positive electrodeactive material in the form of a composite, such as by mixing in amortar with pestle or by a ball-milling process.

According to a particulate and preferred embodiment of the presentinvention, the mixing step is performed by ball-milling.

The step of mixing, and in particular of ball-milling, is preferablycarried out with an inert gas (i.e. under inert atmosphere) such asargon or nitrogen, and preferably in a glove box filled with said inertgas. According to a particulate embodiment of the present invention,argon is preferred.

The step of ball-milling is preferably performed at a temperatureranging from 20 to 300° C., and more preferably from 25 to 80° C.Indeed, this ball-milling temperature is inherent to ball-millingprocess and no external source of heating is used to provide suchtemperatures.

According to an even more preferred embodiment of the present invention,the ball-milling step is carried out in a hard steel ball-miller jarcontaining a weight of milling-balls (W_(mb)) such as the weight ratioW_(mb)/W_(s), with W_(s) being the total weight of powder materialscontained in the jar (Na₃P powder, Na-based positive electrode activematerial and optionally powder of an electronically conductive agent),ranges from about 10 to 60, preferably from about 20 to 60, and morepreferably from about 30 to 50.

The volume of solid materials into the ball-miller is preferably ⅓ lowerthan the volume of the ball-miller jar.

The process according to the invention can be carried out in aball-miller operating by vibrating movements of the balls in the threespatial directions or in a ball-miller operating by centrifugingmovements of the balls.

As an example of ball-miller operating by vibrating movements of theballs, one can mention the ball-miller sold under the reference 8000M bySpex® comprising a metallic jar having an intern volume of 30 cm³ and avibration frequency set at 875 cycles/minute (clamp speed).

As an example of ball-miller operating by centrifuging movements of theballs (planetary ball-miller), one can mention the ball-miller soldunder the reference PM 100 by Retsch. This ball-miller operates at aspeed ratio of 1/(−1) and a rotation speed up to 1000 rotations perminute (rpm). In this type of ball-miller, grinding is essentiallycarried out thanks to the balls that crush the powders and solidsagainst the inner wall of the jar. Grinding is therefore essentiallycarried out by pressure and friction. The combination of impact forcesand friction forces ensures a very high and efficient degree of grindingof planetary ball-millers.

When the ball-milling step of the process of the invention is performedin a ball-miller operating by centrifuging movements of the balls, therotation speed is preferably set at a value ranging from about 200 and1000 rpm, and more preferably from about 400 and 650 rpm.

The duration of the ball-milling step may vary depending on the rotationspeed set for the ball-miller and on the amount of solid materials togrind. In order to avoid a temperature rise, the ball-milling step canbe performed in several grinding sequences, said sequences beingseparated by breaks allowing the decrease of the temperature inside thejar. As an example, when a Spex® 8000M or Retsch PM 100 ball miller isused, the ball-milling step can be carried out according to a sequenceof alternating series of 30 minutes of grinding and 15 minutes of break.

In said ball-millers, the effective duration of the ball-milling step(not including breaking times) can vary from about 0.1 to 5 hours,preferably from about 0.2 to 2 hours, and more preferably from about 0.3to 0.9 hour.

The molar ratio Na₃P/Na-based positive electrode active material capableof inserting sodium ions reversibly used in the method of the presentinvention can generally vary from about 0.01 to about 1, and preferablyfrom about 0.05 to about 0.5.

The positive electrode composite material obtained at the end of theprocess can be used immediately or stored, preferably under an inertatmosphere.

It comprises at least one Na-based positive electrode active materialcapable of inserting sodium ions reversibly and Na₃P, preferably in theform of an intimate mixture.

The Na-based positive electrode active material capable of insertingsodium ions reversibly is as defined in the present invention.

The positive electrode composite material obtained according to theprocess defined as the first object of the present invention can be usedas positive electrode active material for Na-ion batteries.

A second object of the present invention is therefore the use of apositive electrode composite material obtained according to the processaccording to the first object of the present invention, as positiveelectrode active material for Na-ion batteries.

A third object of the present invention is a positive electrode for aNa-ion battery composed of an electrode material and a currentcollector, wherein said electrode material comprises a positiveelectrode composite material as obtained according to the processdefined by the first object of the present invention.

In a preferred embodiment of the present invention, the amount ofpositive electrode composite material varies from 60 to 100 w %, andmore preferably from 80 to 95 w % with regard to the total amount of theelectrode material.

In addition to the positive electrode composite material, the electrodematerial may comprise one or more binders conferring mechanical strengthto the electrode material.

The binder is preferably a polymer which has a high modulus ofelasticity (e.g. of the order of several hundred MPa), and which isstable under the temperature and voltage conditions in which thepositive electrode is intended to operate. The binder maintains adhesionof the positive electrode material to the current collector, maintainionic contact, and facilitate the formation of a stable interface withthe electrolyte.

The binder may be selected from fluoropolymers such as poly(vinylidenefluoride) (PVDF) or poly(tetrafluoroethylene), cellulose fibers,cellulose derivatives such as starch, carboxymethyl cellulose (CMC),diacetyl cellulose hydroxyethyl cellulose or hydroxypropyl cellulose,styrene butadiene rubber (SBR) and a mixture thereof.

The amount of binder may vary from 0 to 40 w %, preferably, from 3 to 10w % with regard to the total weight of the positive electrode material.

In addition to the positive electrode composite material, the electrodematerial may also comprise at least one electronically conductive agent.The conductive agent may be carbon black, Super P carbon black,acetylene black, ketjen black, channel black, natural or syntheticgraphite, carbon fibers, carbon nanotubes, vapor grown carbon fibers ora mixture thereof.

The amount of electronically conductive agent may vary from 0 to 40 w %,preferably, from 5 to 15 w % with regard to the total weight of thepositive electrode material.

The current collector may be composed of a conductive material, moreparticularly of a metallic material which may be selected fromaluminium, copper, steel and iron.

A fourth object of the present invention is a battery using sodium ionsas electrochemical vector (Na-ion battery), said battery comprising:

-   -   at least one positive electrode,    -   at least one negative electrode,    -   said positive and negative electrodes being separated by an        electrolyte comprising at least one sodium salt,

wherein the positive electrode is as defined in the third object of thepresent invention.

The active material used for the negative electrode can be a selectedamong any negative active materials having large irreversible capacitiessuch as carbon materials, in particular carbon nanofibers or carbonfelt, which may also act as current collector.

According to a preferred embodiment of the present invention, the activematerial of the negative electrode is a carbon material.

The electrolyte of the Na-ion battery according to the invention isgenerally a solution comprising a salt of sodium and a solvent.

The solvent may be a liquid solvent, optionally gelled by a polymer, ora polar polymer solvent which is optionally plasticized by a liquid.

The proportion of liquid solvent in the solvent may vary from about 2%by volume (corresponding to a plasticized solvent) to about 98% byvolume (corresponding to a gelled solvent).

The a sodium salt is preferably selected from the group consisting ofNaClO₄, NaBF₄, NaPF₆, Na₂SO₄, NaNO₃, Na₃PO₄, Na₂CO₃, sodium salts havinga perfluoroalkanesulfonate anion, sodiumbis(perfluoroalkanesulfonyl)methane, sodiumtris(perfluoroalkanesulfonyl)methane and sodiumbis(perfluoroalkanesulfonyl)imide (e.g. NaTFSI, also known under thechemical name sodium bis(trifluoromethanesulfonyl)imide).

The liquid solvent may be composed of at least one polar aprotic solventselected from cyclic and linear carbonates (e.g. ethylene carbonate,propylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, dipropyl carbonate, ethyl methyl carbonate, vinylenecarbonate, fluoroethylene carbonate), esters, nitriles (e.g.acetonitrile, benzonitrile), nitro derivatives (nitromethane,nitrobenzene), amides (e.g. dimethylformamide, diethylformamide,N-methylpyrrolidone), sulfones (e.g. dimethylsulfone, tetramethylenesulfone), sulfolanes, alkylsulfamides (tetraalkylsulfonamides havingfrom 5 to 10 carbon atoms), partially hydrogenated hydrocarbons, cyclicand linear ethers (e.g. diethyl ether, tetrahydrofuran, dimethoxyethane,dioxane, glyme, dimethyltetrahydrofuran, methyl), polyethylene glycolethers (e.g. RO(CH₂CH₂O)_(n)R′ in which R and R′ are CH₃ or C₂H₅ and1≤n≤12), tetraalkyl sulfamides (e.g. RR′NSO₂NR″R′″ in which R, R′, R″,and R′″ are CH₃ or C₂H₅), 3-methyl-1,3-oxazolidin-2-one, and cyclicesters (e.g. γ-butyrolactone).

Said liquid solvent may optionally be gelled by addition of a polymerobtained, for example, from one or more monomers selected from ethyleneoxide, propylene oxide, methyl methacrylate, methyl acrylate,acrylonitrile, methacrylonitrile, N-vinylpyrrolidone and vinylidenefluoride, said polymer having a linear, comb, random, alternating, orblock structure, and being crosslinked or not.

When the electrolyte is a liquid electrolyte, said liquid impregnates aseparator. The separator may be a conventional polymer-based separatorsuch as a Celgard® separator or a Whatman® borosilicate glass fiberseparator.

In a preferred embodiment, the electrolyte is a solution comprising asalt of sodium and one or more carbonates selected from ethylenecarbonate, propylene carbonate, dimethylcarbonate, vinylene carbonate,and fluoroethylene carbonate.

A Na-ion battery according to the invention may be composed of a singleelectrochemical cell comprising two electrodes (i.e. one positiveelectrode and one negative electrode) separated by an electrolyte; or ofa plurality of chemical cells assembled in series; or of a plurality ofchemical cells assembled in parallel; or of a combination of the twoassembly types.

The present invention is illustrated in more detail in the examplesbelow, but it is not limited to said examples.

EXAMPLE 1 Preparation of Na₃P/Na₂Fe₂(SO₄)₃ Composite Material byBall-Milling

In this example, a Na₃P/Na₂Fe₂(SO₄)₃ positive electrode compositematerial has been prepared.

1) Preparation of Na₂Fe₂SO₄)₃

Firstly, Na₂Fe₂(SO₄)₃ was prepared by solid state reaction of Na₂SO₄ andFeSO₄ according to the method disclosed by Yamada A. (NatureCommunications, 2014(5), 4538).

2) Preparation of Na₃P by Ball-Milling

Stoichiometric amounts of metallic sodium as bulk (1.38 g, Sigma) andred phosphorus (0.62 g, Alfa, 325 mesh) were filled into a hard steelball-milled jar of a Spex® 8000M ball-miller (30 cm³) in an Ar-filledglove box and equipped with seven hard steel balls each having a weightof 7 g and a diameter of 12 mm. These solid materials were ball milledfor 2-10 h to obtain Na₃P particles.

3) Preparation of Na₃P/Na₂Fe₂(SO₄)₃ Composite Material

0.8 g of Na₂Fe₂(SO₄)₃ obtained according to step 1) above and 0.05 g ofNa₃P obtained according to step 2) above were filled into a hard steelball-milled jar of a Spex® 8000M ball-miller (30 cm³) in an Ar-filledglove box and equipped with four hard steel balls each having a weightof 7 g and a diameter of 12 mm. These materials were ball milled for 0.3h to obtain particles of Na₃P/Na₂Fe₂(SO₄)₃ composite material.

EXAMPLE 2 Preparation of a Full Cell with Na₃P/Na₂Fe₂(SO₄)₃ CompositeMaterial as Positive Electrode Material

In this example, Na₃P/Na₂Fe₂(SO₄)₃ composite material prepared accordingto example 1, has been used as positive electrode active material toassemble a Na-ion battery comprising carbon nanofibers as negativeelectrode and 1M NaClO₄ in ethylene carbonate (EC)/dimethylcarbonateDMC)(1:1 by volume) as liquid electrolyte.

1) Preparation of Carbon Nanofibres (CNFs)

CNFs were produced by electrospinning using polyacrylonitrile asprecursor according to the method disclosed by Kim J K. (Advanced EnergyMaterials, 2014, 4, 1301448.)

2) Assembly of the Na-Ion Battery

CNFs and Na₃P/Na₂Fe₂(SO₄)₃ composite material were respectively used asnegative and positive electrodes, in a coin cells to evaluate theelectrochemical performance of Na-ion batteries. A Na foil was used asthe counter electrode, with 1M NaClO₄ in EC/DMC (1:1 by volume) as theliquid electrolyte and glass fiber (Whatman®, GF/D) as the separator.The cells were tested in a VMP system (Biologic S.A., Claix, France).

For comparison purpose, a comparative Na-ion battery not forming part ofthe present invention has also been prepared using the same amount ofNa₂Fe₂(SO₄)₃ alone as positive electrode active material in place of theNa₃P/Na₂Fe₂(SO₄)₃ composite material prepared according to example 1.

3) Cycling Performances

Each battery was sealed and then a reducing current corresponding to arate of 0.2 C was applied. The capacity was calculated based on theweight of Na₂Fe₂(SO₄)₃.

For each Na-ion battery, the cycling performances were studied. They arereported on FIG. 1 annexed on which the voltage (in V) is a function ofcapacity (in mAh/g).

These results show that there is large irreversible capacity in thefirst cycle for the battery comprising only Na₂Fe₂(SO₄)₃ as positiveelectrode active material (battery not forming part of the presentinvention) which has a capacity of about 88 mAh/g with about 50%coulombic efficiency in the 1st cycle. The majority capacity loss isattributed to the SEI formation on the carbon electrode which consumedNa. For the battery according to the invention, i.e. comprisingNa₃P/Na₂Fe₂(SO₄)₃ composite material, the results show the capacity wasincreased from 43 to 74 mAh/g demonstrating that the use of such acomposite material makes it possible to fight against irreversiblecapacity in Na-ion batteries.

EXAMPLE 3 Preparation of Na₃P/Na₃V₂(PO₄)₃ Composite Material byBall-Milling

In this example, a Na₃P/Na₃V₂(PO₄)₃ positive electrode compositematerial has been prepared.

1) Preparation of Na₃V₂(PO₄)₃

Firstly, Na₃V₂(PO₄)₃ was prepared by solid state reaction according tothe method reported by J Yamaki et al. (Journal of the ElectrochemicalSociety, 2010, 157(4), A536-A543).

2) Preparation of Na₃P/Na₃V₂(PO₄)₃ Composite Material

1.5 g of Na₃V₂(PO₄)₃ and 0.2 g of Na₃P (prepared according to the methodgiven in step 2) of example 1) were filled into a hard steel ball-milledjar (30 cm³) (Spex® 8000M) in an Ar-filled glove box and equipped withsix hard steel balls each having a weight of 7 g and a diameter of 12 mmNa₃P/Na₃V₂(PO₄)₃ composite material was obtained after ball milling for0.5 h.

3) Electrochemical Performances

The electrochemical performance of Na₃P/Na₃V₂(PO₄)₃ composite materialthus obtained were tested in a Na cell identical to the one used inexample 2 with Na₃P/Na₃V₂(PO₄)₃ and Na metal as working and counterelectrodes, respectively. The charge/discharge curve in the 1st cycle isshown in FIG. 2 annexed (Voltage (V) as a function of Capacity (mAh/g)).The Na that extracted from first plateau at about 1.6V can be used forcompensating Na loss in SEI formation.

EXAMPLE 4 Preparation of Na₃P/Na₃V₂(PO₄)₂F₃ Composite Material byBall-Milling

In this example, a Na₃P/Na₃V₂(PO₄)₂F₃ (NVPF) positive electrodecomposite material has been prepared (denoted Na₃P/NVPF).

1) Preparation of Na₃V₂(PO₄)₂F₃

Firstly, NVPF was prepared by solid state reaction of NaF and VPO₄according to the method reported by Croguennec L et al. (Chemistry ofMaterials, 2014, 26, 4238).

2) Preparation of Na₃P/Na₃V₂(PO₄)₂F₃ Composite Material

0.8 g of NVPF and 0.07 g of Na₃P (prepared according to the method givenin step 2) of example 1) were filled into a hard steel ball-milled jar(30 cm³) (Spex® 8000M) in an Ar-filled glove box and equipped with fourhard steel balls each having a weight of 7 g and a diameter of 12 mmNa₃P/NVPF composite material was obtained after ball milling for 0.5 h.

3) Electrochemical Performances

The electrochemical performance of Na₃P/NVPF composite material thusobtained were tested in a Na cell identical to the one used in example 2with Na₃P/NVPF and Na metal as working and counter electrodes,respectively. The charge/discharge curve in the 1st cycle is shown inFIG. 3 annexed (Voltage (V) as a function of Capacity (mAh/g)). Theseresults demonstrate that the additional charge capacity in the 1st cyclecontributed by Na₃P could be used for compensating Na consumed in SEIformation in a full cell.

EXAMPLE 5 Preparation of a Full Cell with Na₃P/Na₃V₂(PO₄)₂F₃ CompositeMaterial as Positive Electrode Material

In this example, Na₃P/NVPF composite material prepared according toexample 4, has been used as positive electrode active material toassemble a Na-ion battery comprising hard carbon as negative electrodeand 1M NaClO₄ in EC/DMC (1:1 by volume) as liquid electrolyte.

1) Preparation of Hard Carbon Anode

Hard carbon was produced by pyrolysis of glucose according to the methoddisclosed by J. R. Dahn et al. (Journal of the Electrochemical Society,2000, 147, 1271.)

2) Assembly of the Na-Ion Battery

Hard carbon and Na₃P/NVPF composite material (prepared according to themethod given in step 2) of example 4) were used as anode and cathode,respectively, in a coin cells to evaluate the electrochemicalperformances of Na-ion batteries. A Na foil was used as the counterelectrode, with 1M NaClO₄ in EC/DMC (1:1 by volume) as the electrolyteand glass fiber (Whatman®, GF/D) as the separator. The cells were testedin a VMP system (Biologic S.A., Claix, France).

For comparison purpose, a comparative Na-ion battery not forming part ofthe present invention has also been prepared using the same amount ofNVPF alone as positive electrode active material in place of theNa₃P/NVPF composite material prepared according to example 4.

3) Cycling Performances

Each battery was sealed and then a reducing current corresponding to arate of 0.2 C was applied. The capacity was calculated based on theweight of NVPF.

For each Na-ion battery, the cycling performances were studied. They arereported on FIG. 4 annexed on which the voltage (in V) is a function ofcapacity (in mAh/g). These results show that there is large irreversiblecapacity in the first cycle for the battery comprising only NVPF aspositive electrode active material (battery not forming part of thepresent invention) which has a coulombic efficiency of about 70% in the1st cycle. The majority capacity loss is attributed to the SEI formationon the carbon electrode which consumed Na. For the battery according tothe invention, i.e. comprising Na₃P/NVPF composite material, the resultsshow the capacity was increased from 89 to 115 mAh/g demonstrating thatthe use of such a composite material makes it possible to fight againstirreversible capacity in Na-ion batteries.

EXAMPLE 6 Preparation of Na₃P/Na_(0.67)Fe_(0.5)Mn_(0.5)O₂ CompositeMaterial by Ball-Milling

1) Preparation of Na_(0.67)Fe_(0.5)Mn_(0.5)O₂

Firstly, Na_(0.67)Fe_(0.5)Mn_(0.5)O₂ (denoted Na_(0.67)FMO) was preparedby solid state reaction according to the method reported by S Komaba etal. (Nature Materials, 2012, 11, 512).

2) Preparation of Na₃P/Na_(0.67)Fe_(0.5)Mn_(0.5)O₂ Composite Material

Two type of Na₃P/Na_(0.67)FMO composite material was produced(Na₃P/Na_(0.67)FMO-1 composite material and Na₃P/Na_(0.67)FMO-2composite material):

i) Preparation of Na₃P/Na_(0.67)FMO-1 composite material: 0.8 g ofNa_(0.67)FMO and 0.09 g of Na₃P (prepared according to the method givenin step 2) of example 1) were filled into a hard steel ball-milled jar(30 cm³) (Spex® 8000M) in an Ar-filled glove box and equipped with fourhard steel balls each having a weight of 7 g and a diameter of 12 mmNa₃P/Na_(0.67)FMO-1 composite was obtained after ball milling for 0.5 h.

ii) Preparation of Na₃P/Na_(0.67)FMO-2 composite material: The contentof Na₃P has been further increased. 0.8 g of Na_(0.67)FMO and 0.18 g ofNa₃P (prepared according to the method given in step 2) of example 1)were filled into a hard steel ball-milled jar with the same ball millingprocess as for the preparation of Na₃P/Na_(0.67)FMO-1 compositematerial.

EXAMPLE 7 Preparation of a Full Cell withNa₃P/Na_(0.67)Fe_(0.5)Mn_(0.5)O₂ Composite Material as PositiveElectrode Material

In this example, Na₃P/Na_(0.67)FMO composite materials preparedaccording to example 6 (Na₃P/Na_(0.67)FMO-1 and Na₃P/Na_(0.67)FMO-2composite materials), have been used as positive electrode activematerials to assemble Na-ion batteries comprising hard carbon asnegative electrode and 1M NaClO₄ in EC/DMC (1:1 by volume) as liquidelectrolyte.

1) Preparation of Hard Carbon Anode

Hard carbon was the same as in example 5.

2) Assembly of the Na-Ion Battery

Hard carbon and each Na₃P/Na_(0.67)FMO composite material were used asanode and cathode, respectively, in a coin cells to evaluate theelectrochemical performance of Na-ion batteries. A Na foil was used asthe counter electrode, with 1M NaClO₄ in EC/DMC (1:1 by volume) as theelectrolyte and glass fiber (Whatman®, GF/D) as the separator. The cellswere tested in a VMP system (Biologic S.A., Claix, France).

For comparison purpose, a comparative Na-ion battery not forming part ofthe present invention has also been prepared using the same amount ofNa_(0.67)FMO alone as positive electrode active material in place of theNa₃P/Na_(0.67)FMO composite materials prepared according to example 6.

3) Cycling Performances

Each battery was sealed and then a reducing current corresponding to arate of 0.2 C was applied. The capacity was calculated based on theweight of Na_(0.67)FMO.

For each Na-ion battery, the cycling performances were studied. They arereported on FIG. 5 annexed on which the voltage (in V) is a function ofcapacity (in mAh/g).

These results show that there is large irreversible capacity in thefirst cycle for the battery comprising only Na_(0.67)FMO as positiveelectrode active material (battery not forming part of the presentinvention) which only has a capacity of 71 mAh/g. For the batteriesaccording to the invention, i.e. comprising Na₃P/Na_(0.67) FMO-1composite material, the results show the capacity was increased from to115 mAh/g, and further increased to 168 mAh/g when Na₃P/Na_(0.67)FMO-2composite material was used as cathode material. These resultsdemonstrate that the use of such a composite materials makes it possibleto fight against irreversible capacity in Na-ion batteries.

1. A method for producing a positive electrode composite material for abattery using sodium ions as electrochemical vector, wherein said methodcomprises at least one step of mixing a powder of Na₃P with a powder ofat least one positive-electrode active material capable of insertingsodium ions reversibly, said step of mixing being carried out in a dryatmosphere and without heating.
 2. The method of claim 1, wherein saidpositive-electrode active material is selected from: lamellarfluorophosphates Na₂TPO₄F in which T represents a divalent elementselected from Fe, Mn, Co, and Ni, and in which T may be replacedpartially by Mg or Zn; fluorosulfates NaT′SO₄F in which T′ represents atleast one element selected from Fe, Mn, Co, and Ni, a part of T′ beingoptionally replaced by Mg, and a part of the sulfate groups SO₄ ²⁻ ofwhich is optionally replaced by the isosteric and iso-charge groupPO₃F²⁻; polysulfides Na₂S_(n) (1≤n≤6), and sodium salts ofdimercaptothiadiazole and of dimercaptooxazole; dithiocarbamatesNa[CS₂NR′R″] in which each of the groups R′ and R″ represents a methyl,ethyl, or propyl radical, or else R′ and R″ form a ring; Na₂Fe₂(SO₄)₃;NaFePO₄, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃; P2 type layered crystallineNa-phases selected from Na_(0.67)Fe_(0.5)Mn_(0.5)O₂, Na_(0.67)MnO₂,Na_(0.74)CoO₂, Na_(0.67)Co_(0.67)Mn_(0.33)O₂,Na_(0.67)Ni_(0.25)Mn_(0.75)O₂ and Na_(0.67)Ni_(1/3)Mn_(2/3)O₂; andNaNi_(1/3)Mn_(1/3)Co_(1/3)O₂.
 3. The method according to claim 1,wherein Na-based positive electrode active material is selected from thegroup consisting of Na₂Fe₂(SO₄)₃, NaFePO₄, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃,Na_(0.67)Fe_(0.5)Mn_(0.5)O₂, Na_(0.67)MnO₂, Na_(0.74)CoO₂,Na_(0.67)Co_(0.67)Mn_(0.31)O₂, Na_(0.67)Ni_(1/3)Mn_(2/3)O₂ andNaNi_(1/3)Mn_(1/3)Co_(1/3)O₂.
 4. The method according to claim 1,wherein the amount of Na₃P varies from 2 w % to 40 w % with regard tothe weight of positive electrode active material.
 5. The methodaccording to claim 1, wherein the mixing step can be performed in thepresence of an electronically conducting agent in powder form.
 6. Themethod according to claim 1, wherein the mixing step is a step ofball-milling.
 7. The method according to claim 6, wherein the step ofball-milling is carried out with an inert gas.
 8. The method accordingto claim 6, wherein the step of ball-milling is performed at atemperature ranging from 25 to 80° C.
 9. The method according to claim6, wherein the ball-milling step is carried out in a hard steelball-miller jar containing a weight of milling-balls (W_(mb)) such asthe weight ratio W_(mb)/W_(s), with W_(s) being the total weight ofpowder materials contained in the jar, ranges from 10 to
 60. 10. Themethod according to claim 6, wherein the ball milling step is carriedout in a ball-miller operating by centrifuging movements of the balls ata rotation speed set at a value ranging from 200 and 1000 rotations perminute.
 11. The method according to claim 6, wherein the effectiveduration of the ball-milling step varies from 0.1 to 5 hours.
 12. Use ofa positive electrode composite material obtained according to theprocess defined in claim 1, as positive electrode active material forNa-ion batteries, said composite material comprising said at least oneNa-based positive electrode active material and Na₃P.
 13. A positiveelectrode for a Na-ion battery composed of an electrode material and acurrent collector, wherein said electrode material comprises a positiveelectrode composite material as obtained according to the processdefined in claim 1, said positive electrode composite materialcomprising said at least one Na-based positive electrode active materialand Na₃P.
 14. The positive electrode according to claim 13, wherein theamount of positive electrode composite material varies from 60 to 100 w% with regard to the total amount of the electrode material.
 15. Abattery using sodium ions as electrochemical vector, said batterycomprising: at least one positive electrode, at least one negativeelectrode, said positive and negative electrodes being separated by anelectrolyte comprising at least one sodium salt, wherein the positiveelectrode is as defined in claim
 13. 16. The battery of claim 15,wherein the active material of the negative electrode is a carbonmaterial.
 17. The battery of claim 16, wherein the carbon material ofthe negative electrode is composed of carbon nanofibers or of a carbonfelt.